The higher number is usually when I'm going through a large number of Non Fiction Books, and depending on the material I like to either pace it out due to intrest or due to the density of the material.

I'll often just read a chapter or two a week, for especially dense books as I like to think through then, and I try to read tem at a time when I can concentrate, so not lunchtime reading.

I'm also usually rereading a couple of old favourites at any given time, as when I'm rushed or tired I often prefer the comfort of an old book, as I can just enjoy the writing without having to worry about understanding or keeping track.